UNIFORM IRRADIATION OF THE CRANIOSPINAL AXIS WITH A PENUMBRA MODIFIERAND AN ASYMMETRIC COLLIMATOR

Purpose: A technique is described that uses an independent, asymmetric collimator and a penumbra modifier to uniformly administer radiation over the craniospinal axis. Method and Materials: An isocenter is used at the junction of the cranial and the upper spinal fields. These fie lds are defined by a single isocenter at spinal cord depth and an inde pendent, asymmetric collimator. From the isocenter, the cranial and up per spinal fields are extended 1 cm inferiorly and superiorly, respect ively. A modifier provides a 2-cm wide penumbra at the central axis of the beam (overlapping region) and is attached to a wedge tray in an a ccessory slot. This modifier allows the fields to be matched so a unif orm dose is delivered over the isocenter and junction. Dose distributi on was measured with an anthropomorphic head-and-neck wax phantom that included the seventh cervical vertebrae. A film was placed in a coron al cut at the spinal cord level. Results: The administered dose varied less than 10% through the craniospinal axis. Reproducibility with por tal films has been very good. Advantages include dose homogeneity, eas y reproducibility, and decreased setup time. Conclusion: This single-f ield isocentric technique allows more uniform irradiation of the crani ospinal axis than do previously described techniques.